Although Z-form DNA is a nucleotide found about 30 years ago, which has a left-handed Z form structure, biological roles of the Z-form DNA have not been well established. The reason is because it is difficult to obtain a stable left handed Z-form of nucleotide in a solution in the case of nucleotides used for studies, which usually have such a short length as 6 to 20 mers. Consequently, development of a monomer unit for stabilizing the Z-form DNA is demanded for studying the Z-form DNA.
The present inventors have previously found and reported in literatures that Z-form DNA can be stabilized by 8-methylguanosine (MeG) (e.g. refer to the prior art reference 1). They have also made numerous studies by using 8-methyldeoxyguanosine (e.g. refer to prior art references 1, 2, 3 and 4).
By using 8-methyldeoxyguanosine, stabilization of the Z-form DNA, which has been extremely difficult to obtain up to now, could be achieved to some extent. However, it is still insufficient to completely stabilize the Z-form DNA, and a further study to search a monomer unit to effectively stabilize the Z-form DNA is still demanded.
Prior art references relating to the present invention are as follows, and they are incorporated herein by reference.    1. Sugiyama, H.; Kawai, K.; Matsunaga, A.; Fujimoto, K.; Saito, I.; Robinson, H.; and Wang, A. H.-J., “Synthesis, Structure and Thermodynamic Properties of 8-Methylguanine-Containing Oligonucleotides Z-DNA under Physiological Salt Conditions.”; Nucleic Acid Res., 1996, 24, 1272.    2. Kawai, K.; Saito, I.; Sugiyama, H., “Conformation Dependent Photochemistry of 5-Halouracil-Containing DNA: Stereospecific 2′-α-Hydroxylation of Deoxyribose in Z-form DNA.”; J. Am. Chem. Soc. 1999, 121, 1391-1392.    3. Kawai, K.; Saito, I.; Kawashima, E.; Ishido, Y.; Sugiyama, H., “Intrastr and 2′-β-Hydrogen Abstraction of 5′-Adjacent Deoxy guanosine by Deoxyuridin-5-yl in Z-form DNA”; Tetrahedron Letters 1999, 40, 2589-2592.    4. Oyoshi, T.; Kawai, K.; Sugiyama, H., “Efficient C2′ α-Hydroxylation of deoxyribose in Protein-Induced Z-form DNA.”; J. Am. Chem. Soc., 2003, 125, 1526-1531.